Various types of microphones are known. Among them, ones used by users holding the housings of microphones are called handheld microphones. The handheld microphone has a microphone unit mounted on the tip of the housing of the microphone.
The handheld microphone may suffer vibration generated by contact of the microphone body with, for example, the hand of a user holding the handheld microphone. The vibration is transmitted to the microphone unit to cause noise. The user may also move while holding the body, and may apply undesirable acceleration to the handheld microphone. Such acceleration also causes noise.
In order to prevent such noise, a typical handheld microphone has a structure appropriately designed for supporting the microphone unit. Such a structure includes a shock mount between the microphone unit and the microphone case (see Japanese Unexamined Patent Application Publication No. 2008-177633). The shock mount can protect the microphone unit from undesirable vibration applied to the microphone case.
The shock mount functions as a suspension. Even if undesirable vibration or acceleration is applied to the body of the handheld microphone, the shock mount can prevent such vibration from propagating toward the microphone unit. This configuration can prevent noise caused by the vibration (vibratory noise).
Unfortunately, the suspension also has an inherent resonant frequency. If the frequency of the undesirable vibration is equal to the resonant frequency, the vibration of the microphone unit is amplified. The suspension is consequently disposed so as to have a lower resonant frequency band than the sound pickup band of the microphone unit. This configuration can reduce contamination of the vibratory noise due to the suspension in the sound pickup band. It is however difficult to set the resonant frequency of the suspension outside the sound pickup band. The resonance of the suspension therefore causes larger vibratory noise than that without any suspension. The resonance of the suspension causes vibratory noise in a low frequency band. This noise is not readily audible to human ears. The resonant frequency of the suspension in a main sound pickup band however leads to larger audible vibratory noise.
A softer suspension can provide a lower resonant frequency of the suspension. A softer suspension however causes the microphone unit to sag due to the gravity and to readily come into contact with the interior of the microphone case. The contact also generates noise. As described above, the handheld microphone has a difficulty in adjusting the suspension so as to provide an appropriate resonant frequency value of the suspension holding the microphone unit and to certainly hold the microphone unit.
FIG. 6 is a graph illustrating example vibratory noise outputted due to vibration applied from a vibrator to a handheld microphone, and example frequency response of the handheld microphone.
In FIG. 6, a dotted line B indicates a frequency response of the handheld microphone. A solid line A indicates vibratory noise outputted from the handheld microphone including a suspension. As indicated by the solid line A in FIG. 6, the vibratory noise increases around 70 Hz, i.e., the resonant frequency of the suspension. The vibratory noise has a lower frequency than the main sound pickup band of the handheld microphone. Such a frequency band may cause a negative effect on the sound quality.
An example approach for reducing such vibratory noise is a reduction in noise component included in the output from the microphone unit through a filter circuit.
For example, a high-pass filter is used which has a higher cutoff frequency than the resonant frequency of the suspension. This configuration can reduce the vibratory noise from the suspension. If the cutoff frequency of the high-pass filter, however, approaches the main sound pickup band (if the cutoff frequency increases to a certain high level), this causes deterioration of the sound quality in the main sound pickup band. Such deterioration of the sound quality may be prevented with a high-pass filter having a higher order. A high-pass filter however cannot provide sufficient attenuation of the vibratory noise level in the resonant frequency of a suspension having a high Q-value.
In another approach, the output from the microphone unit is processed with a notch filter as an active filter. Only the main sound pickup band can thereby be extracted to reduce the vibratory noise. The active filter is however composed of complicated circuitry, readily causes distortion, and cannot provide a sufficient dynamic range.
For example, the output from the microphone unit may also be processed through the notch filter or passive filter. FIG. 3 illustrates example output (frequency characteristics) from the microphone unit through the notch filter or passive filter. In FIG. 3, the horizontal axis represents frequency while the vertical axis represents the output level of the microphone. As illustrated in FIG. 3, the output can be attenuated in the frequency band (about 70 Hz) of the vibratory noise amplified by the resonance of the suspension. However, the output level around 100 Hz decreases in response to sufficient attenuation of the vibratory noise. That is, the vibratory noise cannot sufficiently be attenuated only through the notch filter as a passive filter.